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Evaluation of the reduction, tightening and gripping performance of an innovative set screw technology for instrumented posterior lumbar fusion: A biomechanical study

Ardura F, Chenaux D, Pascal-Moussellard H, Hessmann MH.

Highlights

  • Convex set screw, shaft tip technology (C-STM) shows significantly better construct alignment and stability under constraint compared to standard set screws.
  • Axial gripping forces being 40% higher for the convex set screw in comparison to the standard flatset screw.
  • C-STM-technology supports controlled fixation in terms of applying appropriate forces for correction or fixation during Pedicle Screw System assembly with friction-reduced final alignment and tightening to avoid unnecessary mechanical stress acting on the spine

Abstract

Background

Instrumented posterior lumbar fusion with top-loading pedicle screw systems (PSS) requires fully tightened set screws to achieve a secure fixation and symmetric load condition. This assumes a complete reduction of the rod by 90°, which is not always attainable in situ, especially under constraint. The objective of this work is to compare the mechanical performance of different innovative set screw technologies, which should improve the tightening process.

Hypothesis

The hypotheses of the study are that modifications to the screw and screwdriver unit can 1) improve the quality of set screw tightening and 2) increase the axial gripping capacity of the construct.

Materials and Methods

The four set screw technologies under investigation include a standard set screw with a flat surface (F-S; control group), a set screw with a convex surface (C-S) and a shaft tip method screwdriver used in combination with both flat (F-STM) and convex set screws (C-STM). The quality of set screw tightening is categorized as follows: failed = the rod is not completely reduced; reduced = the rod is successfully reduced but the set screw is not correctly fixed; good = remaining cases. An axial gripping capacity test is performed by a universal testing machine (Instron®) with a force capacity of 5kN.

Results

Regarding the quality of set screw tightening, comparisons between F-S vs. F-STM, F-S vs. C-STM and between C-S vs. C-STM show statistically significant differences (p<0.001). The axial gripping capacity test shows mean gripping forces of 1223N (STD 331) in the F-STM group and of 1724N (STD 168) in the C-STM group with statistically significant differences between both groups (p=0.003).

Discussion

Several biomechanical and clinical case studies have identified possible effects of misaligned rod-screw interfaces such as screw pull-out during rod reduction, screw loosening, screw or rod breakage, misalignment, adjacent segment degeneration and worsening of the clinical outcome.

C-STM-technology thus supports controlled fixation in the sense of applying appropriate forces for correction or fixation during PSS assembly as well as friction-reduced final alignment and tightening with the aim to reduce implant loosening, hardware failure and reoperations, while respecting anatomical and biomechanical balance.

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